<p>Simulations of biologically realistic neurons in large densely connected networks pose many problems to application programmers, particularly on distributed memory computers. We discuss simulations of hundreds of thousands to millions of cells in a model of neocortex in the context of new computing platforms with many tens of thousands to hundreds of thousands of processing elements. We are developing a performance model for this simulation so that we can gauge its performance on these platforms in terms of memory usage, time to setup and execute the simulation, and to estimate the practical limits to the size and simulation timescale available to us in our simulation experiments. Recent results from runs on a BlueGene/L computer, which could ultimately scale to over one hundred thousand processors, are described.</p>